Recent advancements in X-ray source technologies have opened up the possibility for directly observing photoinduced chemical reactions as they unfold on the femtosecond time scale. An increasing number of time-resolved X-ray scattering experiments are being directed toward uncovering the light-induced ultrafast dynamics of photocatalytic metal complexes in solution. In this scenario, theory and modelling are brought into play to offer assistance to the interpretation and analysis of intricate measured data. Besides, theoretical modelling is the key to the fundamental understanding of the atomistic mechanisms behind reaction dynamics in solution.

The work presented in this thesis deals with extending, benchmarking and applying a novel multiscale atomistic modelling strategy for simulating the structural dynamics of complex molecular systems. The method is based on thedirect Born-Oppenheimer Molecular Dynamics (BOMD) propagation of the nucleiand treats solvent effects within a quantum mechanics/molecular mechanics(QM/MM) framework.

The first part of the thesis shows how the QM/MM scheme is augmented toinclude electronic excited states with arbitrary spin multiplicity using a ΔSCFapproach. We describe the testing and implementation of the method in theGPAW DFT code, providing all prerequisite theoretical background. The robustness of the implementation and the computational expediency of GPAWallow fast configurational sampling, overcoming the problem of statistical accuracy in excited-state BOMD simulations of systems as large as transition metal complexes.

The second part is dedicated to an investigation of the structure and dynamicsof a model photocatalyst, the diplatinum(II) complex [Pt2(P2O5H2)4]4-, abbreviated PtPOP. In doing that we make extensive use of the computational tools presented in the first part. First, we show how ΔSCF for the first time provides computational evidence that the lowest-lying singlet and triplet excited states have parallel potential energy surfaces (PESs) along the Pt-Pt coordinate. Then we highlight the synergy between time-resolved experiments and simulations in unravelling the photoinduced ultrafast dynamics of the complex in water.QM/MM BOMD simulations are used to guide the analysis of X-ray DiffuseScattering (XDS) data measured at an X-ray free electron laser (XFEL), and toelaborate a semi-classical picture of ground-state hole dynamics that explainsthe experimental outcome. Finally, we take a step forward in the understandingof the excited-state vibrational relaxation in solution. We show, through thesimulations, that PtPOP after excitation does not retain the symmetry of theground state, as so far believed; and that excess Pt-Pt vibrational energy is firstdirected towards vibrational modes involving the ligands, while the role of thesolvent is to favour intramolecular vibrational energy redistribution (IVR) inthe complex.